1. Field of the Invention
The present invention relates to an optical system, in particular to a projection objective of a microlithographic projection exposure apparatus, and a method for improving imaging properties of such an optical system.
2. Description of Related Art
Optical assemblies comprising at least one movable optical element are known on the market, and include projection objectives for microlithographic projection exposure apparatus. With these and other sophisticated optical systems a high image quality is required in order to produce a picture of a structure that is as free of image defects as possible. The movability of at least one optical element within such a projection objective lens system serves to vary the imaging properties of the projection objective lens system with the aim of reducing occurring image defects.
The choice of the position to which a moveable optical element should be adjusted for reducing image defects has hitherto often been made by individual measurement of the imaging properties of the optical elements before they are assembled. Since many image defects are produced only during assembly, for example as a result of stress induced by lens mounts, such an approach has proven to be too inaccurate.
There are other approaches in which the positioning of the optical elements is improved on the basis of readily visualisable target quantities that reproduce, although only incompletely, the image quality and that have been obtained from the interaction of the optical elements. These approaches rely on the experience of the technician entrusted with the adjustment of the assembly to find the most favorable rotational position. Such optimization methods are insufficiently deterministic.
A method that necessarily leads to a very good or even optimum relative position between the optical elements takes measurements of the image defects of the optical assemblies, including both moveable and stationary optical elements, at all possible positions of the movable optical element. This procedure is too tedious and complicated since the number of possible positions is very large, and therefore measurements are extremely time consuming.
In the attempt to improve projection objectives in order to satisfy increasingly stringent requirements as regards image quality, projection objectives have been proposed in which movable optical components can be arranged at various positions within the projection objective system. In this connection the number of movable optical components is not limited to one; instead there may often be several movable optical elements within the projection objective lens system.
With such projection objectives the question arises at which position should a movable optical element be provided within the projection objective in order to be able to correct a specific image defect, and how many optical elements may optionally have to be moved for this purpose. In addition there is the question, what degree of freedom of movement can be employed in order to correct a specific image defect. Such degrees of freedom of movement include the rotation of optical elements within the projection objective lens system, the displacement of optical elements along the optical axis of the projection objective lens system (focusing) and vertical thereto (centering), and the tilting of optical elements within the projection objective lens system.
Thus there exists a plurality of degrees of freedom that are in principle available for correcting image defects within a projection objective lens system.
With the previously known optical systems a choice of the degrees of freedom that were employed for correcting image defects was made on the basis of trial-and-error methods. In the same way as when finding the most favorable rotational position, here too the experience of the respective technician was decisive in finding useful degrees of freedom, which however led to adjustment results that were not deterministically reproducible. Often the choice of the lenses to be moved as well as the choice of the degrees of freedom of movement were very time-consuming and also did not always achieve given specifications.
Sometimes it is in principle known which lenses within a projection objective lens system have to be moved in order to correct specific image defects. But also in these cases a multidimensional problem exists if several lenses within a projection objective lens system can be moved. As a result an optimal position configuration of all movable lenses in which the overall image defect usually falls below given specifications, or an absolute minimum cannot be found with reasonable effort and expenditure.
U.S. Pat. No. 6,934,011 B2, which is a continuation of U.S. Pat. No. 6,678,240 B2, discloses a method for optimizing the imaging properties that successfully overcomes the above-mentioned problems. According to this known method, the overall image defect of an optical system comprising at least two optical elements is measured and represented as a linear combination of base functions of an orthogonal function set. Then the spatial relation of the at least two optical elements is changed. The overall image defect is measured again in this new spatial relation and represented as a new linear combination of base functions. From these representations the image defect of each of the at least two optical elements is calculated. This makes it possible to determine a target position of the at least two optical elements in which the overall image defect is minimized.
The present invention is based on this known method, but seeks to further improve the results that may be obtained with this known method. For example, it is difficult with this known method to correct image defects having a higher azimutal order. Such image defects may be caused by lenses or other optical components having significant inhomogeneities of the refractive index and/or non-rotationally symmetric surface defects.